I've seen that done both ways. It seems the new way is to do it the easiest way.
I think it should match the trim face color.

 

Your house, your call.

 

Any trim that goes up against siding like that against lap siding is going to be basically the same - not just window siding. It's typical for the edges of trim to be painted the same color as the siding because it's very tedious and time consuming to follow a jagged contour with a brush (if brushing) or to tape off like that (if spraying). When brushing it that way, you don't have to follow any line at all - you just brush outward and the edge comes out perfect without even trying. If you got a quote for painting your house, and then asked them to paint the edges of the trim white, they would but would probably charge more.

Without taking that into consideration, from a purely aesthetic point of view, painting it white would look better. But it gets very little attention usually.

 

painting it white would look better

30' away you won't be able to see what it's painted!

 

While the correct traditional way is to paint the sides of the trim with the window trim color it is quicker/easier to paint it like yours was done. It's all a matter of time/money and how much of either the homeowner wants to spend.

 

If diy, paint the trim color. If contract, more pros will take the easy way, if they are pros. Could be former day laborers and they always take the easy way. If you're not there to oversee, expect to get the easy way.
That photo trim end grain looks already weather damaged. I primed and one coat of finish paint on wood trims. While waiting to dry, I probably would put 2nd coat of primer on face. When cut, sealed the ends with good caulk and 2nd coat of paint. Butt joints were caulked first.